A radar monitoring system according to U.S. Pat. No. 5,268,698 includes several emitters arranged around an airport area and some receivers in the vicinity of a control tower. An aircraft or any other vehicles moving in the airport area is located by comparing signals received by the receivers with a grid of the area when it is unoccupied and by triangulation.
Another radar monitoring system is known from European patent 0,744,630. It includes radar sensors disposed along a runway and generating substantially overlapping beams. A central unit processes radar data transmitted by the radar sensors and receives via a transponder identification information regarding the aircrafts to produce a map of the runway where the locations of stationary objects, such as buildings, and aircrafts that are landing and taking off in the vicinity of the runway are identified. An intrusive threat detected in the monitored area is transmitted to the aircrafts by means of telemetry interfaces and the transponder.
European patent 0,550,073 discloses a radar monitoring system for aircrafts running in an airport area including at least three emission and reception radar stations arranged at the corners of a quadrilateral airport area and illuminating at least one sector of 90° in azimuth. For instance, through bistatic measurements of the received signals and selection of different frequencies of the emitted signals, no shade area, such as one created by a building, remains in the monitored area.
All these radar monitoring systems do not precisely locate a unidentified small object placed on a runway.